Different design modifications of the point-like leading end of a drilling head of the above specified kind are discussed in Metals Handbook (Ninth Edition), Vol. 16, pp. 226-227, 1989, USA. The particular type of the drill point design to which the present invention refers is shown in FIG. 23 on page 227 of this Handbook.
According to this particular design of the drilling head, each of its chip evacuation flutes has a first side wall facing in the direction of rotation of the drilling head, a second side wall diverging outwardly from the first side wall, and a web-thinning recessed surface formed therebetween adjacent the leading end of the drilling head and extending towards this leading end, so that the web thickness, defined at the leading end of the drilling head by the distance between the web-thinning recessed surfaces, is minimal. Each land of the drilling head is bound by the first wall of one adjacent flute and the second wall of another adjacent flute, and has a generally radially extending and axially inclined front flank face, which tapers towards the leading end of the drilling head. The front flank face of each land intersects the associated first wall of the flute at a primary cutting edge, intersects the associated web-thinning recessed surface of the same flute at a secondary cutting edge, and intersects the front flank surface of the other land at a chisel edge which is disposed at the leading end of the drilling head and passes through the axis of rotation of the twist drill.
With the above geometry of the leading end of the drilling head, the width of the chisel edge is essentially reduced when compared with other drill point designs, whereby the thrust exerted on the leading end of the drilling head during drilling is reduced and the cutting efficiency at the center of the drilling head and the centering capability thereof are improved.
Drilling heads similar to that described above are disclosed in further detail in EP 320 881 and KR 96-9591.
However, since it is practically impossible to provide an ideal point chisel edge, i.e. having a zero width, the above drilling heads still encounter problems connected with their centering capability as well as with their cutting at the center of the drill.
It is therefore the object of the present invention to provide a new drilling head having advantages in the above respect.